She's using a Samsung S5 using Android 6.0.1. I'm using a Samsung Note 4 using Android 6.0.1. I've looked at both settings for the Messenger app, and they're the same (the default). The only thing I can think that would make her phone different than anyone else's is that we're both on the same family account with T-Mobile.
All of this said, I have installed and tested MightyText which is a very similar app, and have gotten exactly the same results. This leads me to believe it's something in the phone configuration.
Any ideas on what might be causing this to happen?